OFDM (orthogonal frequency division multiplexing) is well known in the art and utilizes a principle of a duality between the time and frequency domains. A data transmission is conventionally made by creating frequency domain symbols around baseband (centered at zero Hz), followed by an IFFT (inverse fast Fourier transform) to convert the frequency domain symbols (or subcarriers) into the time domain. Next, a guard interval (also known as a cyclic prefix) is placed on the time domain block, and the block is up-converted and transmitted. The up-conversion is conventionally from baseband to an RF band with a center frequency and a bandwidth that are pre-known to both the transmitter and the receiver. At the receiver, the block of data is down-converted back to baseband, followed by a removal of the guard interval, followed by a FFT (fast Fourier transform), followed by equalization to produce the original frequency domain symbols. Pilot subcarriers are inserted to assist equalization. FEC (forward error correction) is normally used against channel errors.
Conventionally, OFDM signals are demodulated by mixing the center frequency to DC (zero Hz) using a complex demodulator, and low-pass filtering all of the sidebands to include all of the component subcarriers, but to exclude the energy that is out-of-band. Mixing and filtering can be either analog or digital, with digital generally being preferred in modern communication systems.
OFDM signals are comprised of frequency-domain subcarriers (or frequency domain symbols), where the number of subcarriers is usually two raised to some integer number, such as 1024, or 4096. In the time domain, component subcarriers are sine and cosine basis functions employing a cyclic prefix (or guard interval) which provides immunity to echoes with an associated time delay. By having an integer number of cycles, the subcarriers of a composite signal maintain orthogonality to each other.
This current technology presents a constraint in modern receiver design, such as cell-phones or terminal units (such as cable modems) where the bandwidth is extremely wide, but at times the receiver needs only a small amount of data, while the receiver must be low cost and/or have low power consumption. This commonly happens in sleep-mode, while receiving a low bandwidth phone call, or in standby. At other times, the receiver needs to receive large quantities of data occupying a large bandwidth, and using more electrical power is acceptable.
If every receiver must demodulate everything to find a small quantity of data for itself, the signal processing wastes energy and shortens battery life.
It is therefore desirable to provide an improved receiver where the above identified problems are alleviated.